The methods for thermal dye transfer developed in the past few years permit the reproduction of an electronically generated image in the form of "hard copy". The principle of such a process is as follows:
A digital image is prepared with the primary colors cyan, magenta, yellow and black and converted into corresponding electrical signals which are then converted into heat by means of a thermal head. Under the action of heat, the dye sublimates from the donor layer of a dye ribbon (color sheet) in contact with the receiving material and diffuses into the receiving layer.
The aim of this technology is to bring the image quality of color prints to the level of silver-salt photography.
To achieve images of high quality with regard to optical density, color tone and resolution, a further requirement must be imposed upon the receiving element in addition to a smooth surface, heat stability, photostability, good dye solubility and "antiblocking" properties, namely, a so-called softness. This property is necessary to produce good contact between the thermal head of the printer and the receiving material. If this is not achieved, unprinted areas or density fluctuations may be observed on the image surface.
The curling behavior of the receiving material represents a further problem. The receiving materials often tend to assume a pronounced curvature after a pass through the thermal printer, which is also retained over the course of time.
The problem of softness can be solved, for example, by the use of an intermediate layer performing the function of a padded layer. Known from JP 02/274,592 is the insertion of an intermediate layer of foamed polypropylene between substrate material and receiving layer.
The same problem is supposed to be solved in JP 03/268,998 by the application of foamed polyolefin layers to both sides of the substrate.
Known from EP 0,551,894 is the application of a composite foil to a substrate, which contains a porous core layer and at least one surface layer which is free of pores.